This invention relates to water-compatible polymers containing both hydrophilic and hydrophobic segments. The hydrophobic segments in turn contain reactive groups. In water, the hydrophobic portions of the polymer tend to associate bringing the chemically reactive groups which they bear in closer proximity than would be expected if the reactive groups were merely randomly distributed within the water. These water-compatible polymers can be further polymerized through the reaction of the chemically reactive groups. This further polymerization can be accomplished by either free radical initiators acting through radiative, redox, or thermal mechanisms or alternatively by ionizing radiation (e.g., electron beam) alone.
The crosslinking characteristics of the polymers of the instant invention can be contrasted to the frequently encountered situation in which polymers are crosslinked in a random fashion. Random crosslinking can occur, for example, either because the initiator or crosslinkable groups are distributed in the polymer in a random fashion. Specific examples include electron beam or ultraviolet irradiation cure of hydrocarbon polymers, hydrogen abstraction from such polymers by benzophene, and vulcanization of natural rubber. The result of random crosslinking is a broad distribution of molecular weight between crosslinks, with the smaller molecular weights segments determining in general the strength of the crosslinked polymer network. That is, as a number average molecular weight between crosslinks decreases, the polymer's modulus increases. See D. W. Van Crevelen, Properties of Polymers, Elsevier, N.Y., 1972 at 161-62. However, as the molecular weights between crosslinks decreases, eventually the point is reached at which the chain statistics of that portion of the chain between crosslinks is no longer well approximated by the classical random flight model as the crosslinked network tends to lose its elastomeric properties. See, for example, J. D. Ferry, Viscoelastic Properties of Polymers, John Wiley & Son, 399-402, (1961). For example, permanent hydrogels prepared from low molecular weight monomers and crosslinkers are generally friable when prepared from low concentrations of monomers and crosslinkers. Furthermore, the monomers used to prepare these permanent hydrogels are generally toxic (e.g., acrylamide). The polymers of the instant invention permit the preparation of networks with a fixed minimum molecular weight between crosslinks. This minimum molecular weight, fixed by the length of the hydrophilic segments, resolves the dilemma encountered by the polymer chemist seeking to prepare a truly elastomeric network of high modulus. This is because the length of the hydrophilic segment can be chosen to be long enough to confer elastomeric properties yet short enough to maximize the density of crosslinks, which in turn is proportional to the modulus.
Linear reactive polymers containing both hydrophilic and hydrophobic segments with reactive groups directly bonded to the hydrophobic portions of the polymer are known in the art. For example, U.S. Pat. No. 3,907,865 discloses linear polymers prepared by reacting polyethylene oxide diols, having molecular weight of less than 500, with diisocyanates and hydroxyethyl(meth)acrylates as well as branched polymers prepared using multi-ols of molecular weight from about 2000 to about 3000 in place of the diols. These polymers can be used with reactive diluent and photosensitizers to produce ultraviolet cured printing plates and release coatings. The molecular weight of the hydrophilic segments of these polymers is restricted to a low value; otherwise, photocuring becomes problematic as the concentration of the reactive groups decreases.
An example of polyethylene oxide-based polymers with a random distribution between crosslinking sites is taught by U.S. Pat. No. 4,047,936. Here copolymers of ethylene oxide and allyl glycidyl ether were prepared giving a random distribution of reactive groups along the polymer. These materials can be contrasted to those disclosed by U.S. Pat. No. 3,867,329 in which polyethylene oxide molecules which have been transesterified give terminal (meth)acrylate groups are described. Although the molecular weight between crosslinks in the corresponding polymer network is fixed by the length of the polyethylene chains, no association of reactive groups occurs prior to formation of the network because these reactive polymers do not contain hydrophobes adjacent to the reactive groups.
Another group of reactive polymers is described in U.S. Pat. Nos. 3,939,105 and 3,939,123. Here reactive polymers are prepared from polyethylene oxide and molecular weight less than 25,000 and diisocyanates such that the reactive groups are terminal residual isocyanate groups. Cure of the reactive polymers dissolved in organic solvent is by reaction with crosslinkers such as organic polyamine in amounts equivalent to the residual isocyanate groups. These polymers, which are not disclosed to be radiation curable, must be stored under anhydrous and inert atmosphere conditions.
Concentration of reactants by absorption or solubilization techniques in micelles is known. For example, see A. Blumstein, "Polymerization in Preoriented Media" in Pasika (Editor), Advances in Macromolecular Chemistry, Vol. 2, Academic Press, New York (1970) and the references in J. H. Fendler, "Interactions and Reactions in Reverse Miceller Systems", Accounts of Chemical Research, Vol. 9, (4), 153 (1976). The ultraviolet polymerization of a reactive monomer in liquid crystalline media containing initiators is also known. Molecular Crystals and Liquid Crystals, Vol. 12, 215-27, (1971). Concentration of reactive monomer into monolayers, e.g., octadecylacrylate at a nitrogen-water interphase, and the polymerization of the monolayer by electron beam radiation is also known. M. Hatada, et al., Macromolecules, Vol. 8 (1), 19-22 (1975).